1. Field of the Invention
The present invention relates to high-frequency transmission lines that transmit high-frequency signals.
2. Description of the Related Art
As a related art of a thin high-frequency transmission line that transmits high-frequency signals, a device having a structure such as described in Japanese Unexamined Patent Application Publication No. 2012-253362 has been proposed. A high-frequency transmission cable described in Japanese Unexamined Patent Application Publication No. 2012-253362 has a so-called triplate stripline structure. Specifically, a first ground conductor, a signal conductor, and a second ground conductor are arranged sequentially along the thickness direction of a plate-shaped dielectric body defining gaps in between. The first ground conductor and the second ground conductor are connected by via-conductor. The second ground conductor includes two elongated conductors extending along a longer-length direction of the dielectric body. The two elongated conductors are formed near both end portions of the dielectric body in a shorter-length direction perpendicular or substantially perpendicular to the longer-length direction. The two elongated conductors are connected to each other by bridge conductors arranged along the longer-length direction defining gaps in between. In this way, a plurality of openings is formed in the second ground conductor along the longer-length direction (extending direction). Forming the openings in this way makes the high-frequency transmission line more flexible.
Further, in order to set the impedance of the high-frequency transmission line to a desired value, it is desirable to suppress capacitive coupling between the signal conductor and the second ground conductor. Thus, the signal conductor and the second ground conductor are formed so as not to overlap with each other in a planar view of the high-frequency transmission line except at the bridge conductors. In the high-frequency transmission line described by Japanese Unexamined Patent Application Publication No. 2012-253362 in which the triplate stripline is adopted as a basic configuration, the following shortcoming may occur in a case where an attempt is made to narrow the width of the high-frequency transmission line.
In the high-frequency transmission line described in Japanese Unexamined Patent Application Publication No. 2012-253362, because of high-frequency signal transmission, when a current (hereinafter, referred to as a signal current) flows in the signal conductor, a return currents are produced in the first ground conductor and the second ground conductor and flow in the opposite direction to the signal current.
Here, in order to make the width of the high-frequency transmission line narrow while making the signal conductor and the second ground conductor not overlap with each other in the planar view as described above, at least one of or both the conductor widths may need to be narrowed. On the other hand, in order to suppress transmission loss of the signal conductor, the width of the signal conductor cannot be narrowed. This inevitably necessitates a reduction in the width of the second ground conductor.
However, as described above, the return current flows in the second ground conductor. As the width of the second ground conductor becomes narrower, transmission loss due to the return current increases. This increases transmission loss of the whole high-frequency transmission line.